In a basic gas monitoring instrument, an electrically powered motor drives a pump to bring a sample of gas from a region or space, typically via a flexible conduit or tubing, to a sensor so that the sample of gas can be tested for a contaminant. The flow rate is affected by the fluid dynamics of the system, which can change from time to time. When the flow through the system is not constant, the gas monitoring instrument is not predictable in its operation. Prior art systems operate the pump at a flow rate greater than required to insure that the minimum required flow rate is obtained. Accordingly, it would be desirable to provide a system wherein a minimum steady flow is maintained in a manner consuming only that amount of energy necessary to maintain the minimum steady flow and even when there is resistance to flow in the system. It also would be desirable to provide such a system wherein a more reliable indication of a low flow condition is given than is provided in existing prior art gas monitoring instruments.
The characteristic response time of the gas monitoring system must be known so that the user can determine when a valid test of the safety or technical compliance of a space has been made. When a potentially hazardous space is monitored, the response time of the monitoring system is a critical parameter for the safety of persons in or entering the monitored space. In some situations, a gas-containing enclosure is monitored for compliance to specific technical requirements. The characteristic flow rate and corresponding response time of a given configuration may be determined by laboratory testing. The measured response times of various configurations are usually repeatable in field applications, provided the tested flow rate is maintained.
In a monitoring instrument having a positive-displacement pump driven by a direct current motor, an obstruction of the gas flow will result in an increased electric current through the motor. Common design practice with such pumps is to sense the motor current and indicate a “low-flow” condition when the current exceeds a predetermined limit. However, this method has a serious drawback. Motor current corresponds not only to flow obstruction, but also to such variables as motor and pump friction, lubricant temperature, diaphragm stiffness, and battery voltage. The uncertainty of the motor current at the desired threshold of minimum flow is so great that it is often necessary to indicate obstructions of the gas flow by detecting short-term increases in motor current. As a result, low-flow is indicated only when rapid decreases of the gas flow occur, such as may result from pinched tubing or pressing the probe against a flat surface. This method leaves the user exposed to the risk of undetected hazards when gas flow decreases gradually, such as by the accumulation of particles in protective filters. Such accumulation may occur over a few minutes or many days, depending on the application.
User safety and confidence may be enhanced by low-flow detection that operates independently of the rate at which gas flow declines. A gas flow sensor placed in the path of the sampled gas could provide the needed indication. Gas flow sensors employ various techniques, including differential pressure across a known flow restriction, rotation rate of a turbine, and thermal convection (mass flow). Such sensors add cost and bulk to the apparatus.